Cumulus clouds are the big puffy clouds that look great in epic musicals and old Dutch paintings. They're the kind of clouds that let you know you're likely to get rained on. So why are they up in the sky over desert wildfires so often?

Wildfires tend to start when grass is incredibly dry and the day is hot. Generally they make the news at the end of a long summer, when there's no rain in sight. Occasionally, though, a little raincloud will form and sit atop the long column of smoke from a wildfire. These puffy white clouds are called pyrocumulus clouds, and often form on top of giant fires. Why?

Pyrocumulus clouds formed the same way large storm clouds are formed - just in miniature. Air from the ground is swept upwards into the sky. Often, air near the ground carries evaporated water, and when it hits the cold, high atmosphere it condenses and turns into a cloud. A fire actually helps this process along by throwing ash into the air. Water vapor condenses on matter, and the ash particulates are seeds, around which droplets can grow. There have been pyrocumulus clouds that have gotten so big that they've turned into thunderstorms.

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For the most part, though, they're just little puffy white clouds that seem to form out of the clear blue sky above fires, begging for a children's book writer to think up a story for them.